Light fuel start-up fluidized bed combustor

ABSTRACT

A fluidized bed combustor having in its distributor plate assembly at the bottom of the combustion chamber, air outlets in the distributor plate surface, air flow riser tubes projecting above said surface, light liquid fuel nozzles located adjacent certain of the riser tubes for injection of liquid fuel into the air forcefully flowing up these riser tubes, and the share of air flowing through the riser tubes relative to the share of air flowing through the outlets being controlled by regulating means.

BACKGROUND OF THE INVENTION

This invention relates to fluidized bed combustion apparatus, andparticularly to a fluidized bed combustor.

Fluidized bed combustion units have been developed heretofore forcombustion of solid particulate fuels such as coal or wood. Other unitshave been developed for liquid fuels such as oil. Normally, all suchunits employ a fluidizable bed material such as silica sand. Althoughcombustion of solid fuel is effective once the unit is operational,start-up can be bothersome. Moreover, if a poor quality coal iscombusted, such as coal having noncombustible components approachingfifty percent or the like, the constant accumulation of thenoncombustion debris is particularly bothersome such that frequentshutdown for clean out is necessary, causing lost time and followed bydifficult start-up.

SUMMARY OF THE INVENTION

A fluidized bed combustor operational on a variety of fuels includingsolid particulate fuels such as poor quality coal, having the capacityfor ignition on light liquid fuel such as alcohol or the like, and ifnecessary or desired, continued operation on the light liquid fuel. Thedistributor plate assembly is specially constructed to enableintroduction of the liquid fuel into the bed through certain air risertubes with a share of the combustion air, another share being throughriser tubes without fuel, and the remaining share being through outletsin the distributor plate. Regulating means is provided to control theshare of total combustion air flow through the riser tubes, and theshare through the outlets in the distributor plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end elevational view of a boiler employing the fluidizedbed combustor of this invention;

FIG. 2 is a side elevational view of the boiler in FIG. 1;

FIG. 3 is a fragmentary elevational sectional view of the combustor ofthis invention;

FIG. 4 is a fragmentary elevational view of the combustor in FIG. 3,also showing the solid fuel feed mechanism and the debris removalmechanism;

FIG. 5 is an enlarged fragmentary sectional elevational view of thelower portion of the combustor;

FIG. 6 is a fragmentary enlarged perspective of a portion of thedistributor plate surface of the combustor;

FIG. 7 is a fragmentary enlarged sectional view of a portion of thedistributor plate in FIG. 6;

FIG. 8 is a plan view of the distributor plate assembly; and

FIG. 9 is a plan view of the debris removal conveyor assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now specifically to the drawings, the boiler assembly 10 (FIG.1 and 2) employing the invention herein includes a pressure vessel 12 ofconventional type. Associated therewith is a typical smoke box 14leading to the exhaust stack 16. The pressure vessel is to contain wateror other heat transfer fluid which may be converted to steam or vapor.It is to include conventional controls such as water gage glass 18, feedwater supply valve 17 with feed water controls, low water cut-off, andhigh and low water alarms 20. It has surface blow off device 22 and amain steam outlet 24. The particular arrangement of the heat exchangerin the boiler can vary considerably and may for example be of the typeshown in U.S. Patent Application Ser. No. 5072 filed Jan. 22, 1979. Theheat for generation of the steam is supplied from the novel fluidizedbed combustor assembly 26 in the lower portion of the apparatus.

In the embodiment shown (FIG. 1), the combustor is composed of threeadjacent fluidized bed zones 28a, 28b and 28c, each capable ofindependent operation. In operation, each bed would include afluidizable bed material such as silica sand or the like. This multipleof fluidized beds is located at the bottom of a combustion chamber 30(FIG. 3), defined by chamber walls 32 which typically would berefractory lined (as depicted in FIG. 3) or water filled (as depicted inFIG. 5). The lower portion of the walls converge downwardly as depicted.

At the bottom of the combustion chamber is the multiple distributorplate arrangement, with one distributor plate assembly for eachfluidized bed. For convenience, only one of these will be described indetail, the others being basically the same. Each distributor plateassembly 32 (FIG. 5) includes a housing 34 which defines a first upperplenum 36 and a second lower plenum 38 separated by a partition 40.Projecting down into the upper plenum 36 is an invertedfrustopyramidally-shaped distributor plate surface area 42 terminatingat its lower end in a central opening 44 communicating with a verticaldrop tube, e.g. 46c depending therefrom for controlled removal of hotdebris from the fluidized bed during operation as described hereinafter.This surface area 42 is specially formed with pierced or punchedelongated slots 49 and integral vertically offset elongated shields 51thereover, the shields being formed by the metal offset from the platesurface during formation of the slots. The long dimension of theindividual slots is oriented toward the central opening 44 in the bottomof the distributor plate, i.e. down the sloping surfaces thereof. Theslots and shields form laterally oriented air outlets. The technique fordisplacing metal plates in this fashion is conventional in the metalworking industry. As will be explained hereinafter, the combustion airflow through these outlets supplies a portion of the combustion air forthe respective fluidized bed, also serving as a portion of thefluidizing air for the bed, and further causing particulateclassification along the sloped surfaces of this surface area. The slotsand outlets communicate with the upper first plenum 36 so that air flowthrough the plenum emerges into the combustion chamber through theseoutlets.

For combustion of some solid fuels, it has been found possible to employa flat distributor plate in place of the sloped frustopyramidallysurfaced type, and even to employ such without the noted slots. However,the capabilities of the structure depicted in the drawings are generallyfar superior.

Projecting upwardly from the lower second plenum 38 (FIG. 5) to anelevated position above the surface 42 is a plurality of riser tubes,usually cylindrical, formed of a first set of riser tubes 50 and asecond set 52. Both sets of riser tubes project substantially above thesurface area 42, with risers 52 projecting above risers 50. Risers 52have air outlet openings adjacent the top thereof located above the airoutlet openings adjacent the top of risers 50. Both sets of risers haveinlet openings at the bottoms communicant with plenum 38 for flow of theother portion of combustion and fluidizing air therethrough into thecombustion chamber. Also adjacent the bottom of risers 52 is a pluralityof respective liquid fuel nozzles 56 supplied by liquid fuel inletconduit 58 for introduction of a light liquid fuel such as alcohol,gasoline, kerosene, styrene, and/or other light liquid hydrocarbonfuels. The nozzles are oriented to spray the fuel onto the interiorwalls of the riser tubes 52 for conductance of the fuel by the risingfilm principle up through the tubes along with the air flowing upwardlytherethrough, and discharge of both into the bed through the sameoutlets.

Air is propelled into the housing under force through inlet 60 (FIG. 5)and in the specific embodiment depicted, into lower plenum 38. Means isprovided for controlling the amount of the air which flows to the risertubes and the amount which flows to the slots in surface 42. In theparticular embodiment depicted, this is done by a pivotably movablebaffle or damper 62 positioned in partition 40 between plenum 38 andplenum 36, to thereby regulate the proportion of the air which flows tothe first plenum and thus out slots 49 as opposed to the portion whichflows from plenum 38 to riser tubes 50 and 52. This proportioningcontrol means could alternatively be positioned upstream of both plenumssuch that the air to plenum 36 passes directly thereto rather thanthrough plenum 38, or the air could be made to flow first to plenum 36,with part of it being controllably discharged to plenum 38. Each bed hasits own such baffle or damper control. Each also has its own air supplymeans such as draft fan 64 (FIG. 4). If desired, additional air outletports 45 can be provided within the drop tube 44 to assist in theclassification function to be described hereinafter.

Temperature measuring elements such as thermocouples 70 projecting up todifferent levels within the bed are employed to monitor the bedtemperature at these levels. These thermocouples are supported by tubes72 (FIG. 5) adjacent the riser tubes for the air and fuel, and connectto temperature monitoring indicators. These sensors can for exampledetect overheating which may result in sintering or glassing of the bedmaterial. Such can activate an alarm, and/or effect safety shutdown.

The drop tube 46a, 46b, and 46c (FIG. 1) for the hot debris includescooling fins 47 (FIG. 5) therearound, in contact with the air flowingthrough plenum 38. This not only maintains the drop tubes within acontrolled temperature range but also preheats the combustion air forthermal efficiency. The lower end of each drop tube has a flange forconnection to an inlet on the debris conveyor apparatus therebelow, anddescribed hereinafter.

Also projecting through the wall of the combustion chamber 30 above theactive bed level AL is an electric spark igniter 71 with controls 73(FIG. 3), and light liquid fuel supply 73a and return 73b.

Monitoring of the bed level is achieved by vertically spaced levelsensors projecting through the wall of the combustion chamber (FIG. 5),there being a lower level sensor impulse port 76, an intermediate levelsensor impulse port 78 normally just below the slumped bed level SL, andan upper level impulse port 80 just above the active bed level AL. Theselevel sensors 76, 78 and 80 are interconnected with transmitters 82 toindicate bed level. If desired, the sensors can be connected toautomation bed level controls. Thus, if one or more of the beds isinactive, such that the bed material from an adjacent active bed slowlymigrates into the lower level inactive bed, the decreased bed level ofthe active bed can cause the inactive bed to be fluidized for a limitedtime, without fuel combustion therein, to redistribute the particulatebed material back to the active bed. Alternatively, the sensing of a lowor high bed can sound an alarm, start or stop under bed debris removalequipment, start or stop fuel in feed to the bed, and/or start or stopthe forced draft fan for the bed.

The main fuel for the fluidized bed combustor is typically a solidparticulate fuel such as coal. This solid fuel is preferably suppliedfrom a fuel hopper 90, optionally including a limestone hopper 92associated therewith for control of sulfur emissions in known fashion.The individual fluidized beds are respectively supplied with fuel fromthis hopper by individual feed augers in housings 92a, 92b and 92c(FIG. 1) driven by appropriate motors 94a, 94b, and 94c, to advance thefeed down appropriate chutes 96a, 96b or 96c, dropping the fuel intoeach bed. In the preferred apparatus, the individual augers powered bymotors 94a, 94b, and 94c are metering screws, with motors 94a, 94b, and94c being of variable speed type. These cooperate with individualconstant speed motor-driven screws, e.g. 98c (FIG. 4) which push thefeed into the individual beds. The fuel feed can be variable speed,intermittent or continuous as necessary.

In the particular embodiment depicted, the hot debris discharged fromthe fluidized beds through the drop tubes enters a water cooled conveyorsystem 100 (FIG. 1, FIG. 9). More specifically, a plurality of conveyors102a, 102b and 102c have upper openings communicating with the lowerends of the drop tubes to receive the hot noncombustible debristherefrom. These conveyors include housings 104a, 104b and 104c (FIG. 9)with coolant flow passages, usually for water, and an auger with coolantflow passages, usually for water. More specifically, (FIG. 8) a coldwater source 105 supplies coolant to line 107 to the individual augerpassages 110a, 110b and 110c, which then flows back from the augers tohousings 104a, 104b, and 104c through lines 106a, 106b, and 106c. Thecoolant discharges through lines 108a, 108b, and 108c to outlet 122.

The hot debris is preferably advanced by these conveyors to a crossconveyor 112 (FIG. 9) which also has a water cooled auger using inletline 114 from source 105, the output from the auger flowing through line116 to water cooled housing 118 to be discharged therefrom through line120 to outlet 122, to be fed to the boiler, if desired, for thermalefficiency.

During operation, assuming hopper 90 contains particulate fuel such ascoal, the individual beds are selectively fluidized by forcefullypropelling air from the draft fans, e.g. 64 into the selecteddistributor plate assemblies and more specifically into the lowerplenums 38 thereof. The individual dampers 62 are regulated to achievedesired air flow proportioning. One portion of the air is propelled intothe combustion chamber out openings 49 and the other portion throughlateral outlets in the tops of riser tubes 50 and 52, to fluidize theparticulate bed. As the bed becomes fluidized, light liquid fuel such asalcohol is injected through line 58 and out nozzles 56 into the loweropen ends of riser tubes 52 for conductance thereof with the air up thewalls of the riser tubes 52 and into the bed where the vaporized fuel isignited by igniter 71. This feeding of the liquid fuel is continueduntil the bed temperature is elevated to a desired range, at which timethe fluidized bed is ready for introduction of the solid particulatefuel from hopper 90. As the solid fuel is selectively introduced byindividual drive motors 94a, 94b and 94c, damper 62 for each is adjustedto optimize the relative air flow through the riser tubes and throughthe slot openings in the distributor plate surface. As combustion of thesolid fuel stabilizes, the supply of light liquid fuel is decreased orterminated, causing continued operation to be solely by combustion ofthe solid fuel. If desired, the system can be automatically shut down inresponse to excess temperatures detected by temperature sensors 70. Thesolid fuel infeed is preferably controlled in response to pressurecontrols of the boiler, i.e. adding more fuel if more heat is required.Combustion of the solid fuel causes creation of ash and concentration ofsmall stones and/or other noncombustible substances, i.e. debris presentin the fuel. This is continuously removable on a controlled basis duringoperation. For example, the unit has been demonstrated to successfullyoperate on poor quality coal having more than fifty percent mineral andash content, yet without the typically expected necessity of shutdownevery few hours to clean out the large quantity of debris. Rather, theunit automatically eliminates the debris on a continued basis. This isdone by the special structure and arrangement of the distribution plateassembly. The controlled portion of air through outlets 49 of the platecauses dynamic, continuous classification of the particulates along thesloped surface area 42, and can be adjusted for the fuel encounteredwithout altering the total combustion air input since the remainder isinjected through the riser tubes projecting above surface 42. The airflow at this inverted frustopyramidal surface causes controlledclassification of the particles based upon size, the larger hotparticles slowly migrating down the slope to the drop tube, and droppingdown the cooled tubes to the water cooled conveyor system. Conveyors102a, 102b, and 102c advance the debris while cooling it further, tocross conveyor 112 which cools and propels it to discharge outlet 113.The finer particles of uncombusted or partially combusted materialsmigrate upwardly into the fluidized bed for complete combustion. Veryfine particles of ash or fuel which may be carried with the exhaust areseparated out of the exhaust by a conventional separator or precipitatorin housing 15 below the smoke box, and recycled back through return line31 (FIG. 2) to the fluidized bed.

The detail features of the invention can be modified somewhat to suitthe particular installation. The invention is intended to be limitedonly by the appended claims and the equivalents thereto.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A fluidized bedcombustor comprising:a combustion chamber; a distributor plate assemblyat the bottom of said combustion chamber; said assembly including ahousing forming a first air plenum and a second air plenum; an invertedpyramidal surface in the top of said housing, projecting down into saidfirst plenum; said surface having slots with offset shields thereaboveforming laterally oriented air flow openings from said first plenum tosaid combustion chamber such that air flow therethrough will causedebris particle classification along said surface as well ascontributing to bed fluidization and supplying combustion air; air flowriser tubes projecting up from said second plenum to a position elevatedabove said surface, having air inlets adjacent the bottom of said risertubes and air outlets adjacent the top thereof; said riser tubescomprising a first set for supplying fluidizing and combustion air tosaid combustion chamber, and a second set; the riser tubes of saidsecond set projecting vertically above the riser tubes of said firstset, and having liquid fuel nozzles for injection of light liquid fuelinto the risers of said second set for elevation and discharge of thefuel with air flow through said second set and into said combustionchamber; forced air supply means to said housing for air flow theretounder pressure; and air flow control means for regulating the relativeflow at said first and second plenums to thereby regulate flow throughsaid slots in said surface relative to flow through said riser tubes. 2.A fluidized bed combustor comprising:a combustion chamber; a distributorplate assembly at the bottom of said combustion chamber; said assemblyincluding a housing forming a first air plenum and a second air plenum;a surface in the top of said housing, having air flow openings in saidsurface from said first plenum to said combustion chamber; air flowriser tubes in addition to said air flow openings, said riser tubesprojecting up from said second plenum to a position elevated above saidsurface, having air inlets adjacent the bottom of said riser tubes andair outlets adjacent the top thereof; said riser tubes comprising afirst set of riser tubes and a second set of riser tubes; fuel nozzlesarranged for injection of light liquid fuel into the risers of saidsecond set for elevation and discharge of the fuel with air flow to saidcombustion chamber; forced air supply means to said housing for air flowthereto under pressure; and air flow control means for regulating therelative flow at said first and second plenums to thereby regulate airflow through said air flow openings in said surface relative to air flowthrough said riser tubes.
 3. The combustor of claim 2 wherein outlets ofsaid riser tubes of said second set are above outlets of said risertubes of said first set.
 4. The combustor of claim 2 wherein said airflow control means is between said first and second plenums.
 5. Thecombustor in claim 2 wherein said fuel nozzles are at the lower ends ofsaid riser tubes of said second set, oriented to inject the liquid fuelinto said riser tubes of said second set.